More than a decade ago, it was realized that a significant amount of redundancy could be eliminated from a television signal by encoding information concerning only those portions of each frame that differed significantly from a preceding frame. See F. W. Mounts, "A Video Encoding System Using Conditional Picture-Element Replenishment", Bell System Technical Journal, Vol. 48, No. 7, September 1969, pp. 2545-2554. The intensity value for the spatially corresponding picture element in the previous frame was used to predict the intensity value of the pel presently being processed. For pels in the moving area, the magnitude of the prediction error was encoded, together with the location of the changed pel.
Motion compensation techniques such as those described in U.S. Pat. No. 3,632,865 issued to B. G. Haskell and J. O. Limb on Jan. 4, 1972, improved on the earlier conditional replenishment work by attempting to determine the amount and direction of displacement that occurred in objects in the picture in the time interval between frames. By so doing, a location in the previous frame which offered an even better prediction of the present intensity value could be selected, reducing the magnitude of the error values to be encoded and enabling still more efficient processing. Recently, displacement estimation techniques have been improved by using a recursive technique to update each estimate based on previous estimates. This technique, described in the application of A. N. Netravali and J. D. Robbins, Ser. No. 21,063, filed Mar. 16, 1979, has been applied to predictive encoding in the pel domain (see application of A. N. Netravali and J. D. Robbins, Ser. No. 21,077, filed Mar. 16, 1979) as well as the transform domain (see application of A. N. Netravali and J. A. Stuller, Ser. No. 27,053, filed Apr. 4, 1979). Multiple predictors utilizing displacement compensation is also described in application of A. N. Netravali and J. D. Robbins, Ser. No. 46,953, filed June 8, 1979.
Notwithstanding the improvements in encoding efficiency enabled by recursive displacement estimation, that technique nevertheless does not take account of spatial and/or temporal changes in illumination that may occur in the pictures being encoded, since the prior art techniques all assume uniform lighting conditions. Furthermore, the above-mentioned displacement compensation techniques each require a fair amount of processing circuitry and can be relatively expensive, since a random access memory is generally required to enable retrieval of intensity values which represent displaced picture elements, and since spatial interpolations must be performed to determine intensity values and intensity gradients at picture locations which lie between the discrete points which correspond to the instants at which the video signal is sampled.
In view of the foregoing, it is the broad object of the present invention to improve the interframe coding of video signals with respect to spatial and/or temporal changes in illumination in the picture being processed. Specific objects include reduction in the prediction error in an interframe coder by anticipating illumination changes alone or in combination with movement of objects in the picture. Also, the ability to process the video signal without the need for random access memories and spatial interpolators is desired.